1. Field of the Invention
The present invention relates to a method for reducing complexity in detection of control messages received by a user equipment, when the control messages are transmitted to a user equipment or a user equipment group in a cellular system.
2. Discussion of the Related Art
In a cellular system, in order to allow user equipments (UEs) to receive data from a base station, the UEs should first transmit a variety of information messages (hereinafter, control messages) such as resource locations, in which their data is present, modulation and coding schemes and Multiple-Input Multiple-Output (MIMO) schemes. Such control messages should be present in each of the UEs which will receive the data from the base station. Similarly, a method for grouping a plurality of UEs into groups and transmitting control messages according to the groups may be considered. At this time, the size of each of the control messages may be changed according to the contents of information and the modulation and coding schemes.
Meanwhile, when a base station transmits control messages having different sizes to UEs, a method for reporting the location information of the control messages in a frame and modulation and coding information in advance and a method which does not report the above information may be considered, respectively.
In the former case, since the UEs know the location of the control messages in the frame and the modulation and coding information, the control messages can be easily detected. However, it is disadvantageous that additional information should be further transmitted.
In contrast, in the latter case, since additional information is not transmitted, overhead does not occur, but each of the UEs should perform blind detection in order to confirm its control messages. That is, all the UEs should perform blind detection with respect to all portions of the frame, in which the control messages are present. Meanwhile, if the number of cases which should be considered when the blind detection is performed is increased, the complexity and time for confirming the control messages are increased and thus the power of each of the UEs may be lost. Accordingly, a method for easily performing blind detection while minimizing overhead should be considered.
The complexity of the blind detection is changed depending on how large control messages are constructed or how many control messages are considered. If all the sizes of the control messages are different from one another, the complexity may be increased. Accordingly, the number of sizes of the control messages is generally limited to a predetermined value. For example, if a total of three sizes including 30 bits, 60 bits and 90 bits are allowed as the sizes of the control messages, the complexity may be decreased. If the basic unit of the size is defined as a Control Block (CB) for convenience, control messages having three sizes of 1CB, 2CB and 3CB may be present (if it is assumed that 1CB=30 bits).
However, even when the sizes of the control messages are set in the CB units, the complexity of blind detection is significantly high. For example, if three types of CBs are present and a total of four control messages are considered, in blind detection, three cases (1CB, 2CB and 3CB) are present when the number of control messages is 1, 32 cases are present when the number of control messages is 2, 33 cases are present when the number of control messages is 3, and 34 cases are present when the number of control messages is 4.
If a total of 120 cases are present and a UE does not have information about the cases, the UE should perform detection a maximum of 120 times in order to confirm whether the control messages of the UE are present and which information is included if the control messages are present. In general, if the number of types of CBs is N and the maximum number of control messages is M, a maximum number of times of performing detection is sum(Nm). Since the number of times of performing detection deteriorates system performance, a method for reducing the number of times of performing detection should be considered.
Meanwhile, frequency reuse is one method for increasing the number of channels per unit area in a cellular system. As a distance is increased, the intensity of a wave gradually weakens. Thus, interference between waves is low at places separated from each other by a predetermined distance or more, and thus the same frequency channel may be used. Using such a principle, the same frequency may be simultaneously used in various places such that subscriber capacity is significantly increased. The effective use of the frequency is called frequency reuse. A unit for distinguishing between places is called a cell (mobile communication cell) and frequency channel switch between cells for holding a call is called handoff. In an analog cellular mobile communication scheme, frequency reuse technology is necessary. A frequency reuse rate is one parameter indicating frequency efficiency in a cellular system. The frequency reuse rate is obtained by dividing a total number of cells (sectors) simultaneously using the same frequency in a multi-cell structure by a total number of cells (sectors) of the overall multi-cell structure.
The frequency reuse rate of a 1G system (e.g., an Advanced Mobile Phone System (AMPS)) is less than 1. For example, in 7-cell frequency reuse, a frequency reuse rate is 1/7. The frequency reuse rate of a 2G system (e.g., a Code Division Multiple Access (CDMA) or Time Division Multiple Access (TDMA) system) is better than that of the 1G system. For example, the frequency reuse rate of a Global System for Mobile communication (GSM) which is a combination of a Frequency Division Multiplexing Access (FDMA) system and a TDMA may reach ¼ to ⅓. The frequency reuse rate of a 2G CDMA system and a 3G WCDMA system may reach 1 such that spectral efficiency is increased and network arrangement costs are decreased.
When all the sectors of one cell and all the cells of one network use the same frequency, a frequency reuse rate of 1 can be obtained. However, when the frequency reuse rate of 1 is obtained in a cellular network, signal reception performance of UEs located on the boundary between cells is decreased by interference from a neighbor cell.
In an Orthogonal Frequency Division Multiple Access (OFDMA) system, since channels are separated in the subchannel units, a signal is transmitted via a subchannel and all channels are not used as in the 3G system (CDMA2000 or WCDMA). Using such features, the throughput of the UEs located at the central area of a cell and the UEs located on the boundary between cells can be simultaneously improved. In detail, the central area of a cell is close to a base station and thus is safe from co-channel interference from a neighbor cell. Accordingly, the UEs located on the central area of a cell may use all available subchannels. However, the UEs located on the boundary between cells may use only some of all available subchannels. In the boundary between neighbor cells, a frequency is allocated such that the cells use different subchannels. Such a scheme is called Fractional Frequency Reuse (FFR).
If FFR is applied to a cellular system and the number of cases considered when blind detection is performed is increased, the complexity and time for confirming control messages are increased and thus the power of a UE is lost. Accordingly, there is a need for a method for easily performing blind detection while minimizing overhead.